1. Field of the Invention
This invention relates to an electromagnetic switch having a stationary magnet with a plurality of legs and a moving armature having a corresponding plurality of legs. The invention particularly relates to the disposition of an air gap between an outer leg of the magnet and a corresponding outer leg of the armature to reduce audible noise from the closed magnet.
2. Description of the Prior Art
Electromagnetic contactors are switch devices which are especially useful in motor-starting, lighting, switching, and similar applications. Typically, a contactor has a fixed magnet and a movable magnet (armature). A relatively large air gap resides between the fixed magnet and armature when the contactor is opened. To close the contactor an electromagnetic coil is energized to electromagnetically accelerate the armature towards the magnet. The electromagnetic acceleration should be of sufficient magnitude to overcome the bias of contact pressure springs and a kick-out spring used to separate the armature from the magnet when energy is removed from the coil.
The armature preferably is provided with electrically conductive contacts. When the coil is energized to cause the armature to contact the magnet, the electrically conductive contacts touch stationary contacts affixed in a switch housing. Contact touching generally provides a circuit closure for energizing a circuit or a load.
Typically, contactors are classified as either DC electromagnet devices or AC electromagnet devices. More recently, however, electromagnets are required to operate with chopped waveform excitation that is neither sinusoidal alternating current nor direct current. Magnets operating with chopped waveform excitation have a great propensity to emit objectionable audible noise.
In alternating current contactors, audible noise is caused by the coil voltage returning to zero at a rate determined by the frequency of the power supply, typically 60 Hz. The noise is caused by the armature movement relative to the magnet due to the varying magnitude of the energization signal. In the past, shading coils have been added to the magnetic circuit to reduce the noise. The shading coils provide a current flow even when the voltage is zero i.e., at each zero crossing of the AC-waveform. The current flow results in a force that keeps the magnet closed and acceptably quiet. The shading coils, however, reduce efficiency and increase cost. In simple, electrical systems it would be advantageous to eliminate the use of a shading coil.
An additional means for reducing noise in an electromagnetic contactor includes high tolerance machining of the contacting faces of the respective legs of the armature and magnet. The high tolerance machining smoothes the respective contacting faces of the armature and magnet thereby eliminating noise which would otherwise be present from the grating of one "rough" face against another. The extra machining which is necessary, however, is prohibitively expensive and time consuming.
In AC and DC energized contactors, a non-magnetic gap is usually added to the path of the magnetic system to limit the residual magnetism that causes magnetic sticking. The gap ensures that residual magnetic flux is low, whereby the kick-out spring can quickly open the switch upon cessation of electrical energy to the coil.
Typically, E-shaped magnets and armatures are used in prior art electromagnetic contactors. When an E-shaped magnet is used in prior art systems, an air gap is added to the magnetic path of the center leg by making that leg shorter than the outside magnet legs. As noted above, this air gap increases the magnetic reluctance of the closed magnetic path thus reducing the residual magnetism. A reduction in residual magnetism makes the kick-out spring more effective for separating the magnets during a contactor opening operation. However, in an E-shaped magnetic member, vibration of the center leg due to the fact that there is some room for movement and caused by a magnetic force on the center legs of the magnet and armature to touch causes deflection of the spine of the E-shaped member of the armature, magnet or both thereby allowing the outer leg pieces of the magnet and armature to wipe against each other. The movement of the outer leg pieces causes noise and also causes the outer leg faces to wear which eventually causes the center leg air gap to disappear and the residual magnetism to increase dramatically.
To eliminate the problem of the noise caused by the bending of the spine of an E-shaped permanent magnet armature and the consequent rubbing together of leg faces, and yet to continue to provide a contactor wherein residual magnetic flux is kept to a minimum, the device of U.S. Pat. No. 4,739,293-Hurley et al. was provided. Hurley, et al. teaches a contactor having an E-shaped magnet and armature wherein the center legs of the magnet and armature each have significant magnetic material removed from their faces leaving complimentary ribs or protrusions which mate or make with each other as the outer legs make or mate with each other. The ribs or protrusions are small enough to magnetically saturate and function as though they were an airgap. The removal of the magnetic material keeps residual magnetism at a minimum allowing the kick-out spring to separate the magnetic members during an opening operation.
When machined properly, the device of Hurley, et al. successfully reduces residual magnetism allowing for separation of the magnetic members by the kick-out spring and decreases audible noise. However, in Hurley et al. each of the opposing faces of the armature and the magnet must be precisely aligned or else relative movement of the armature with respect to the magnet will cause objectionable audible noise. The precise machining required of the device disclosed in Hurley, et al. increases the cost of manufacture.
There is a need, therefore, to provide an electromagnetic contactor having low audible noise and low residual magnetism thereby allowing the magnet/armature combination to separate quickly and efficiently during an opening operation.